Fuel tanks on vehicles are subject to a number of safety requirements. These requirements include a venting system for relieving pressure in the tank when such pressure exceeds a predetermined level and a thermal relief system for preventing explosions when the temperature in the tank becomes excessive.
One known type of venting system is a pressure relief valve having a valve plug that is positioned outwardly of a vent opening in the valve housing and is spring biased to close the opening. A valve stem is attached to the plug and extends inwardly therefrom into the tank. The inner end of the stem carries a spring abutment for one end of a compression spring. The other end of the spring abuts the valve housing around the vent opening. The spring is calibrated to allow the valve plug and stem to move outwardly to relieve pressure when the pressure exceeds a predetermined level and to reseat the valve plug to close the vent opening when the pressure drops back down to an acceptable level. This valve arrangement has the advantages of simplicity of construction, reliability, and low cost.
One approach to providing thermally activated pressure relief is the use of a fusible material that melts at elevated temperatures to cause a vent opening or openings to be uncovered. The fusible material may itself be the cover for the opening, or it may be used to attach a nonfusible material cover to another portion of the venting structure. The latter arrangement has the advantages of allowing a relatively large opening to be uncovered very quickly and of requiring only small amounts of fusible material.
In known tank safety systems which have a pressure relief valve and thermal relief based on the melting of fusible material, the relief valve and thermal relief are provided separately. The separate provision of pressure activated relief and thermal activated relief tends to complicate the overall structure of the safety system.
Tank pressure relief valves having a spring biased valve plug and a stem which carries a spring abutment, as described above, are disclosed in U.S. Pat. Nos. 1,994,770, granted Mar. 19, 1935, to G. W. King; and 3,918,606, granted Nov. 11, 1975, to R. D. Keller. Other types of pressure relief valves are disclosed in U.S. Pat. Nos. 2,489,787, granted Nov. 29, 1949, to E. W. Knowlton; and 4,458,711, granted July 10, 1984, to F. S. Flider. The Knowlton and Flider devices also include fusible portions that cover vent openings and melt to uncover such openings. Flider discloses a vent valve with a plastic housing which has a thin walled break-away portion to provide an enlarged aperture under severe over-pressurization and which melts under severe fire conditions to provide an even larger aperture for maximum venting. U.S. Pat. No. 3,669,302, granted June 13, 1972, to M. Markarian, discloses an electrolytic capacitor that has a vent opening sealed by a fusible material.
C. J. Green discloses valves for relieving pressure and preventing liquid spillage in U.S. Pat. Nos. 4,325,398, granted Apr. 20, 1982; 4,457,325, granted July 3, 1984; and 4,487,215, granted Dec. 11, 1984. Each of these three patents also discloses the use of fusible material for attaching a portion of the valve housing that covers a large vent opening to the rest of the housing.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.